[PATCH] fs: consistently deref the files table with rcu_access_pointer()

[Mateusz Guzik <mjguzik@gmail.com>]

... except when the table is known to be only used by one thread.

A file pointer can get installed at any moment despite the ->file_lock
being held since the following:
8a81252b774b53e6 ("fs/file.c: don't acquire files->file_lock in fd_install()")

Accesses subject to such a race can in principle suffer load tearing.

While here redo the comment in dup_fd() as it only covered a race against
files showing up, still assuming fd_install() takes the lock.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
---

I confirmed the possiblity of the problem with this:
https://lwn.net/Articles/793253/#Load%20Tearing

Granted, the article being 6 years old might mean some magic was added
by now to prevent this particular problem.

While technically this classifies as a bugfix, given that nothing blew
up and this is more of a "just in case" change, I don't think this
warrants any backports. Thus I'm not adding a Fixes: tag to prevent this
from being picked by autosel.

 fs/file.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 6c159ede55f1..52010ecb27b8 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -423,17 +423,25 @@ struct files_struct *dup_fd(struct files_struct *oldf, struct fd_range *punch_ho
 	old_fds = old_fdt->fd;
 	new_fds = new_fdt->fd;
 
+	/*
+	 * We may be racing against fd allocation from other threads using this
+	 * files_struct, despite holding ->file_lock.
+	 *
+	 * alloc_fd() might have already claimed a slot, while fd_install()
+	 * did not populate it yet. Note the latter operates locklessly, so
+	 * the file can show up as we are walking the array below.
+	 *
+	 * At the same time we know no files will disappear as all other
+	 * operations take the lock.
+	 *
+	 * Instead of trying to placate userspace racing with itself, we
+	 * ref the file if we see it and mark the fd slot as unused otherwise.
+	 */
 	for (i = open_files; i != 0; i--) {
-		struct file *f = *old_fds++;
+		struct file *f = rcu_access_pointer(*old_fds++);
 		if (f) {
 			get_file(f);
 		} else {
-			/*
-			 * The fd may be claimed in the fd bitmap but not yet
-			 * instantiated in the files array if a sibling thread
-			 * is partway through open().  So make sure that this
-			 * fd is available to the new process.
-			 */
 			__clear_open_fd(open_files - i, new_fdt);
 		}
 		rcu_assign_pointer(*new_fds++, f);
@@ -684,7 +692,7 @@ struct file *file_close_fd_locked(struct files_struct *files, unsigned fd)
 		return NULL;
 
 	fd = array_index_nospec(fd, fdt->max_fds);
-	file = fdt->fd[fd];
+	file = rcu_access_pointer(fdt->fd[fd]);
 	if (file) {
 		rcu_assign_pointer(fdt->fd[fd], NULL);
 		__put_unused_fd(files, fd);
@@ -1252,7 +1260,7 @@ __releases(&files->file_lock)
 	 */
 	fdt = files_fdtable(files);
 	fd = array_index_nospec(fd, fdt->max_fds);
-	tofree = fdt->fd[fd];
+	tofree = rcu_access_pointer(fdt->fd[fd]);
 	if (!tofree && fd_is_open(fd, fdt))
 		goto Ebusy;
 	get_file(file);
-- 
2.43.0